Indoles are expensive but important compounds for use in the preparation of perfumes and as materials for the synthesis of tryptophan, medicines, agricultural chemicals and the like.
Indoles are contained in coal tar, coal-liquefied oil and the like in relatively large quantities. A liquid-liquid extraction method has been proposed as an industrial scale process for the recovery of indoles from these indoles-containing mixtures. In this instance, glycols such as ethylene glycol and ethanolamines such as monoethanolamine have been reported as useful extracting reagents having a high extraction selectivity of indoles.
Since the extraction selectivity of indoles is not satisfactorily high even by the use of such extracting reagents, a process has been studied and proposed in which these extracting reagents are used jointly with aliphatic hydrocarbons such as heptane with the aim of increasing partition coefficient of indoles and, at the same time, decreasing partition coefficients of impurities. A purification process has also been proposed in which impurities in an indoles-containing extract are removed by the extraction with aliphatic hydrocarbons such as heptane.
The term "extraction selectivity is high" as used herein means that a compound of interest in an extraction system has a high partition coefficient compared to those of incidental impurities.
The following summarizes typical processes for the extraction of indoles proposed on the basis of aforementioned studies.
(1) An indole-containing mixture is contacted with an aliphatic hydrocarbon, such as n-heptane, and monoethanolamine to extract indole from the mixture into monoethanolamine layer. The resulting extract (monoethanolamine layer) is then subjected to a distillation step directly or after extracting and removing impurities from the extract by contacting it with an aliphatic hydrocarbon such as n-heptane. Thereafter, a monoethanolamine azeotropic fraction and an indole fraction are obtained by means of rectification. [Koks i Khimiya, 1981 (5), 37-41].
(2) Indole is extracted with ethylene glycol from a mixture containing indole by contacting the mixture with ethylene glycol. The resulting extract (ethylene glycol solution) is contacted with an aliphatic hydrocarbon such as n-heptane to extract impurities from the extract. After the removal of impurities, the resulting raffinate (ethylene glycol solution) is contacted with diisopropyl ether to extract indole with the ether. The resulting extract (diisopropyl ether solution) is subjected to distillation to distill off diisopropyl ether and then washed with water to remove ethylene glycol. Thereafter, an indole fraction is obtained by means of rectification. [U.S. Pat. No. 2,837,531 (1958)].
Each of the aforementioned prior art processes, however, has some disadvantages.
For example, since monoethanolamine has a low selectivity of indole extraction, large quantities of aliphatic hydrocarbons are inevitably used in the aforementioned process (1) in order to improve the extraction selectivity. Especially, large amounts of aliphatic hydrocarbons which exceed the amount of a feed oil as a starting material are required when indoles are extracted selectively from the feed containing a low level of indoles. In consequence, this process has a problem of requiring considerably high costs for the recovery of aliphatic hydrocarbons by distillation and for the installation of equipment.
In the case of the process (2), ethylene glycol for use in the extraction of indoles has a considerable mutual solubility with the other extraction solvent, diisopropyl ether. Consequently, not only the recovery yield of these solvents is low but also the process requires additional treatment steps for the removal of contaminated solvents and the like, thus resulting in the considerably high production cost. A disadvantage of this process is that ethylene glycol has an insufficient selectivity of indole extraction.
The indoles extracted by these processes (1) and (2) contain a significant amount of organic sulfuric compounds such as benzothiophenes, resulting in disadvantages as goods on the market. Therefore, it is necessary to apply additional and costly indole-purification processes.
This invention contemplates overcoming the foregoing problems involved in the prior art.